1. Field of the Invention
The present invention relates generally to telephone accessories and more particularly to a miniaturized direct access coupler for connecting a piece of terminal equipment to a telephone line pair.
2. Description of the Prior Art
A recent Federal Communication Commission (FCC) ruling permits the use of direct access couplers approved pursuant to FCC Ruling Part 68 for use in coupling terminal equipment to the telephone network. Typically, approved prior art couplers include a ring detection relay having a coil which is connected by a DC blocking capacitor and a current limiting resistor to a plug for connection to the tip and ring line of the telephone line pair. The relay is adapted to respond to the 40 to 150 volt RMS 16 to 68 Hertz ring signal developed between the telephone line pair and to energize an associated set of relay contacts which may be used by the associated terminal equipment to recognize a call.
The coupler further includes an off-hook relay having a coil energized by the terminal equipment and having contacts which connect the plug to the primary winding of an isolating transformer, the secondary winding of which may be connected to the terminal equipment to couple AC signals between the telephone line pair and the equipment. When the off-hook relay contacts are closed, the primary winding of the isolation transformer additionally provides a path for current induced to flow by the 42.5 to 52.5 volt DC potential normally developed between the telephone line pair to power associated telephone devices and to notify the telephone company of the off-hook state.
Although this current need not exceed 20 milliamps in order to apprise the telephone company, it is possible for currents approaching 120 milliamps to flow through the primary winding of the primary transformer. It is no doubt apparent that a transformer suitable for sinking this level of DC current while providing coupling for AC signals at a distortion level required for reliable data transmission is relatively large.
In order to reduce this current-caused distortion and also the size of the isolation transformer, some prior art direct access couplers incorporate a DC blocking capacitor connected in series with the transformer and a parallel connected choke connected so as to shunt the off-hook current. Although reducing the distortion and permitting the use of a smaller transformer, the choke is itself relatively large.
In addition to the obvious advantages of incorporating the direct access coupler within the associated terminal equipment (preferably directly upon one of the printed circuit boards) such incorporation may also avoid the need for duplicate circuitry. In particular, where foreign equipment may be connected to a stand-alone coupler, the FCC requires level control circuitry within the coupler designed to insure that signals coupled to the telephone line pair do not exceed maximum signal levels, even though such circuitry may already be incorporated within the terminal equipment.
Finally, direct access couplers are required to withstand certain potentials in excess of the 264.5-volt peak potential developed by the combination of the DC potential and the ring signal. Specifically, the FCC requires that the controller withstand an 800-volt peak potential developed between the tip and ring lines in order to provide a degree of protection in the event that lightning should strike the telephone line pair, and a 1000-volt RMS potential developed between the telephone line pair and other points.